Spain's government plans to force Europe's central bank into sharing the task of bailing out its troubled financial sector in a potentially controversial move that could spark objections from the German chancellor, Angela Merkel.

Spain is considering proposals to inject €19bn (£15bn) of capital into nationalised Bankia in the form of government debt that could then be used to raise money from the European Central Bank (ECB), forcing it to get involved in what may become a far wider bailout of Spain's creaking banking sector.

Details remain sketchy, but sources in Madrid confirmed that a refinancing involving the ECB was the most probable way forward for a Spanish government that will have trouble raising €19bn itself at a manageable interest rate.

By avoiding the markets altogether, the government would indirectly "push the financing of Bankia's bailout on to the ECB", according to El País newspaper.

But the debt scheme raises questions about how the ECB, Merkel and the financial markets might react to Europe's central bank helping rescue a nationalised Spanish bank laden with toxic real estate.

"You will have to ask the ECB that," said one official in Madrid.

With Spain now key to the future survival of the euro, the news of a probable debt-for-shares deal with Bankia may add further tension to the markets on Monday.

Part-nationalised Bankia, which holds 10% of Spanish deposits, on Friday asked the government for €19bn on top of the €4.5bn provided three weeks ago. That made it Spain's biggest-ever bank rescue.

But growing uncertainty about the euro and worries about Spain as it nosedives into a second recession in three years mean the country must now pay above 6% interest on money borrowed for 10 years.

If Spain's conservative government can bail Bankia out by giving it debt, it may be tempted to use the same mechanism with other struggling banks. That would allow it to avoid a politically embarrassing bailout by the European Stability Mechanism – which the French president, François Hollande, has already said is needed.

Although Spain has successfully refinanced debt and covered its budget deficit so far this year, it may soon come under pressure to raise significant extra sums.

Regional governments, for example, must refinance €36bn by the end of the year. But they are priced out of the market, with regions such as Valencia already given junk status by ratings agencies.

Doubts about the regions produced jitters on world markets on Friday after the Catalan president, Artur Mas, appealed for the government to cover regional refinancing by issuing state-backed bonds.

The Catalonia news "implies that the Spanish government may have to take on more debt and it cannot afford to do so," Richard Franulovich, a currency expert at Westpac Securities, told Reuters.

Spain's boast that it had raised half of its 2012 financing now looks fragile. Its long-term advantage in Europe, due to a low level of public debt four years ago, is also slipping away. A budget deficit of 8.9% last year has forced Spain to borrow significant extra sums to pay its bills.

The prime minister, Mariano Rajoy, has imposed a fierce austerity programme to bring the deficit down to 3% next year. That has deepened recession and helped push unemployment close to 25%.

The government already expects to inject up to €15bn into banks this year as they set aside €82bn against toxic real estate assets that have slumped in value since a 2008 housing bust. An independent audit of banking assets may uncover further problems, forcing the government to raise more money to help banks.

Bankia revised its 2011 accounts on Friday and found an extra €15.6bn in provisions it must make against potential future losses. It is not clear how Bankia would use government debt to raise money from the ECB. Bankia could use government bonds "as guarantees on interbank operations, go with them directly to the ECB to get money or, in the worst scenario, sell them on the market," El País reported.

In recent weeks Rajoy's government has insisted that the ECB is the solution to Spain's liquidity problems.

If Spain is unable to use government debt to recapitalise banks and cannot raise money on the markets, it may still have to turn to Europe's bailout funds.

"Spain cannot do this alone," said Luis Garicano of the London School of Economics on a blog posting after Bankia asked for €19bn. "I do not understand why the government is waiting to ask for help from Brussels."

British Airways departure

British Airways is expected to lose its largest shareholder after Bankia said it was planning to sell its stakes in several leading businesses. Bankia, which held a controlling share in Iberia before it merged with British Airways, now holds 12.5% of stock in parent company International Airlines Group (IAG). As it prepares to receive a €23.5bn (£18.7bn) cash injection, however, Bankia will be forced into a fire sale by European authorities, competition regulators who typically require bailed-out banks to sell non-core operations and halt dividend payouts until taxpayers have been repaid. Ailing Bankia has 5.3% of energy firm Iberdrola, 10% of hotel company NH and 18% of SOS, a food company. It also has holdings in real estate companies Realia and Metrovacesa.